Comminution apparatuses are used to refine material, such as fibrous material, grains, non-fibrous materials, and other materials, typically by grinding them into smaller fibers, pieces, or components. Disc refiners are a type of comminution apparatus known in the art for processing many materials from a course grade to a finer grade and even into individual fibers. For example, in the papermaking industry wood chips or other raw fiber stock materials are ground into smaller chips or mechanically treated so that the chips may be broken down further and refined into individual fibers. After refining these stock materials, the individual fibers are typically used to make paper related products, such as sheet paper, toilet paper, paper towels, and other absorbent products. In other industries, the treated individual fibers are utilized to produce certain other products as well. Other such industries are known and include, for example, plastic, thermoplastic and recycled plastic products. Disc refiners in general are utilized to brake down clumps of fibers into individual fibers. Such a disc refiner typically includes a pair of opposed and disc-shaped comminution elements referred to as refiner discs. Each disc typically includes one or more disc-shaped steel or steel-alloy castings having an array of generally radially extending ridges or refiner bars protruding from a refining face or surface of the disc. Refining discs may be formed of one or more continuous annular discs or annuli, or may instead be formed from a plurality of disc segments arranged on the disc relative to one another to form one or more rings or annuli.
One of the pair of opposed refiner discs is mounted on a rotor for rotation therewith and the other disc is mounted on a separate mounting surface which is either fixed or rotatable. The discs are typically opposed so that their refining surfaces face one another defining a gap between them. The refining surfaces are generally placed very close to one another, typically spaced apart from each other no farther than about 0.040 inches. The second refining disc may be attached to a fixed surface of the refiner or to a separate rotor for rotation therewith so that the pair of refiner discs rotate simultaneously relative to one another. As the material to be refined enters the gap between the discs, the relative motion between the refining surfaces breaks down the stock material to a desired degree.
It is desirable that the stock material to be refined remain between the refining surfaces of the two discs long enough to achieve a desired degree of refinement or fiber development. In order to facilitate easy repair and replacement of the refining surfaces, a plurality of annular discs are preferred in the art. Each disc can therefore include a plurality of annular discs or annuli arranged radially spaced apart from one another on the disc. Each disc annulus typically confronts on a facing disc an identical disc annulus whereby the two confront and rotate with respect to one another during operation of the refiner. The disc annuli typically axially taper toward a refining plane between the opposed discs in a direction moving from the radial inner edge to the radial outer edge of each disc annulus. Each opposing disc annulus also typically tapers in the same fashion or at the same angle toward the refining plane as its confronting annulus.
High consistency refiner discs that are generally flat suffer from operational problems that relate to high rotational speeds, large disc diameters, and high power or load input characteristics. The high rotational speed, typically about 1200 rpm or higher, significantly reduces the time the fiber material resides within the gap between the opposed discs and disc annuli. The time the material stays between the discs is referred to as residency time. The reduced residency time and high rotational speed produce a high intensity refining or development and fiber cutting of the stock material. However, the reduced residency time limits further fiber development, desired for producing strong fibers, that typically only occurs when residency time is increased.
The large disc diameters typically associated with these types of discs also reduce the residency time of the stock material between the discs. This is caused by the relationship between disc diameter and the angular acceleration of the stock material between the discs. More specifically, a disc having a larger diameter will impart a greater angular acceleration to the material as the material moves farther radially outwardly relative to the disc than a disc having a smaller diameter. This increased angular acceleration typically causes steam evacuation problems and undesirably high steam pressures within the refining zone or gap between the discs. Steam evacuation problems can cause steam build up which, in turn, can lead to vibration and upsetting of the refiner discs. Vibration and upsetting of either disc causes the refining gap between the discs to frequently vary during operation. This can cause the discs to clash or contact one another and can produce pulp of poor quality after refinement.
Because of the resistance to steam evacuation from the refining gap between the discs, an undesirably high steam pressure can develop between the discs that directly opposes the thrust urging the discs toward each other. Therefore, a greater thrust is needed than is expected without steam in order to produce an equivalent horsepower with high consistency refiners. This phenomenon becomes a problem when the mechanical limitations of the refiner do not allow for the proper or additional thrust force necessary to apply an adequate amount of refining power to the fibers of the stock material.
In a flat disc refiner, residency time is to some degree controlled by upraised dams that are disposed between elongate upraised bars of a refiner disc that define grooves through which the stock material flows. The dams help control residency time by obstructing the flow of stock material through the grooves, requiring the stock material within the refining zone to find alternative paths to the outer diameter of the discs. The use of such dams to control the residency time, however, also undesirably further increases the steam pressure within the refining zone because of the obstruction to the flow of steam as well as stock material between the discs.
What is needed is a disc-shaped comminution element for a comminution apparatus that preferably is a disc for a disc refiner that increases residency time for an element of a given diameter and when operated at high rotational speeds. What is further needed is a refiner disc that has a configuration that minimizes steam build up between opposed discs for minimizing vibration and upsetting of the discs. What is still further desired is a refiner disc that increases residency time while minimizing the use of dams.